Jon-Erik Hexum, costar of CBS' "Cover-Up" series, remained in critical condition yesterday in a Beverly Hills hospital. . .

Hexum accidentally shot himself in the right temple with a prop gun loaded with blanks Friday afternoon during a break in filming of the series at the 20th Century-Fox studios. . .

Hexum, 27, was taken to Beverly Hills Medical Center. . .

He was on a set depicting a hotel room and was holding a .44-caliber magnum between takes when he accidentally shot himself, apparently in view of several crew members. . .

Hexum, who costars with Jennifer O'Neill, plays Mac Harper, a male model turned secret agent, on the series. . .

Overnight figures for the Bush-Ferraro debate Thursday night indicate it attracted a combined 42.6 Nielsen average and 60 percent share of the audience that night on the three networks. . .

That would appear to be a larger audience than the first Reagan-Mondale debate Oct. 7 attracted . . . but the nine-city Nielsen overnight survey may not reflect the final, national averages. . .

For instance, for the first Reagan-Mondale debate, overnights in eight Nielsen cities gave the three-network cumulative audience as 41.1/65 . . . but the national figures totaled 46.7/71 . . . which researchers indicated meant about 70 million viewers. . .

Likewise, the Oct. 7 overnights gave the post-debate analyses on the three networks a 39.3 rating . . . with ABC first, followed by CBS and NBC. . .

But the final national Nielsen figures for the Oct. 7 post-debate shows added up to a 42.5 cumulative rating and put CBS first, followed by ABC and NBC. . .

Regardless, for the Bush-Ferraro debate, overnight figures suggested that ABC had the largest audience, averaging a 14.9 rating and a 20 share, compared with a 13.9/20 for NBC and a 13.8/20 for CBS. . .

But final, national figures for Thursday's post-debate analysis race (which Nielsen totes up quickly, because there are advertisers involved) show CBS won between 10:30 and 11 p.m., with a 14.0/23, followed by ABC with a 12.4/21 and NBC, 11.9/20 . . . suggesting CBS may have won the debate race, too. . .

(As further evidence of the disparity between overnights in the big cities and national figures . . . at least in regards to political programming . . . the Thursday overnights had ABC winning the analysis race, with a 13.2/21, followed by CBS with an 11.4/18 and NBC with an 11.3/18). . .

In Washington Thursday night, WDVM (CBS) drew the largest Bush-Ferraro numbers, with a 21.3/28, followed by WRC (NBC) with a 16.9/22 and WJLA (ABC), 13.8/18. . .

However, ABC sneaked into second place locally for the post-mortem, as WDVM led with a 14.8/24, followed by WJLA with a 13.3/20 and WRC, 11.7/19. . .

I don't know about you, TV Column fans, but Captain Airwaves is all confused. . .

The nine Nielsen markets include, besides Washington, New York, Los Angeles, Chicago, Philadelphia, San Francisco, Boston, Detroit and Dallas. . . Moving Right Along

Can We talk big bucks? . . .

CBS/Broadcast Group is now spending $1.5 billion a year for TV programming . . . which figures out, if sun spots haven't affected the Airwaves Abacus this morning, to about $4.1 million a day, every day. . .

You might want to keep that in mind as you ponder Advertising Age's annual survey of advertising rates, which makes CBS' "60 Minutes" on Sunday night the most expensive prime-time show this new season, with ABC's "Dynasty" on Wednesday a close second. . .

The average 30-second spot on "60 Minutes" costs advertisers $190,000, compared with "Dynasty's" $187,000. . .

Third-priciest program this fall is NBC's Tuesday night entry, "The A-Team," which commands $167,000. . .

CBS top money-makers also include Wednesday's "Simon & Simon," $153,000; Friday's "Dallas," $143,000; Thursday's "Knots Landing," $135,000 and "Magnum, P.I.," $133,000; Friday's "Falcon Crest," $121,000; plus Monday's "Kate & Allie" and "Newhart," both $120,000. . .

Other big earners for ABC include Wednesday's "Hotel," $145,000; NFL Monday Night Football and the Sunday movie, both $140,000; Wednesday's "Fall Guy," $125,000; and Sunday's "Hardcastle & McCormick," $120,000. . .

Befitting the third-ranked network, NBC's most-expensive airtime going into the season was considerably less expensive than its rivals', including Tuesday's "Remington Steele," $124,000; Thursday's "Cheers," $120,000; Tuesday's "Riptide" and Sunday's "Knight Rider" and movie, all at $117,000; and Thursday's "The Cosby Show," $116,000. . .

Even before the early ratings generally confirmed their judgment, advertisers gave the weakest votes of confidence to ABC's "Ripley's Believe It or Not," the network's Sunday competition for "60 Minutes," which goes for $73,000; Friday's Hawaiian Heat," $75,000; Tuesday's "Who's the Boss?" $81,000; and Saturday's "T.J. Hooker," $82,000. . .

CBS' cheapos include Tuesday's duo of "AfterMASH" and "E/R," both $80,000; Wednesday's movie, $83,000, and "Dreams," $84,000; and Saturday's "Cover-Up," $85,000 . . . while both Friday's "Dukes of Hazzard" and Saturday's "Air Wolf" are in for a will-they-be-around-in-January $88,000. . .

NBC's lowest earners include its Sunday competition for "60 Minutes," "Punky Brewster" going for $59,000 and "Silver Spoons," $65,000; Friday's "Miami Vice," $71,000; Saturday's not-so-"Hot Pursuit," $76,000 and "Partners in Crime," $80,000. . .

(If there's a real bargain in the NBC lineup, it may be Michael Landon's "Highway to Heaven," which is performing well so far and cost advertisers only $85,000 per 30 seconds before the season got under way). . .

The first 18 episodes of "It's Academic" on Channel 4 this season are being closed-captioned for the hearing-impaired. . .

Giant Food Inc., the sponsor, is picking up the tab for the service, which is being handled by the National Captioning Institute out in Virginia. . .

That terrible Billy Clyde Tuggle (played by Matthew Cowles) is scheduled to break out of prison this week and return to Pine Valley on ABC's "All My Children" . . . and that definitely spells trouble for tycoon Adam Chandler and his wife Erica, a couple of real sweethearts themselves. . .

Billy Clyde has only served three of the 25 years he got for kidnaping and attempted murder. . .

And from our That Must Have Been A Tough Saturday, Unseen Viewer file: Unseen Viewer phoned yesterday to complain that 1) Channel 20 has "totally lost control" of decibel levels as it switches from programming to commercials lately and that 2) Channel 7 definitely needs some Apostrophic Advice, after airing a crawl Saturday evening, as the Oklahoma-Texas game wound down, that read: " 'Larry King: Lets Talk' will be seen in it's entirety" after the game's conclusion. . .

Sen. Frank Lautenberg (D-N.J.) will be linked with all 64 cable systems in his home state tonight when he holds a one-hour "town meeting" during which viewers can participate in a call-in poll by dialing two 900 numbers. . .